1. Field of the Invention
The present invention relates to a transmission device and a transmission method that are capable of performing wireless communication.
2. Related Background Art
Conventional wireless communication systems can maintain the signal quality thereof by being allocated dedicated frequency bands so that they do not interfere with each other. However, the frequency resources are limited; thus, the case where a plurality of systems (radio access schemes) shares the same frequency band is being considered in order to use the frequency resources freely and more efficiently. In such systems, however, it is necessary to maintain the signal quality by avoiding or suppressing interference among the spectrum sharing systems.
The existing spectrum sharing techniques are classified broadly into underlay spectrum sharing and opportunistic spectrum sharing. For underlay spectrum sharing, the interference that is caused in other incumbent systems that already use the same frequency band is maintained to a noise level or lower by dispersing the power of a signal over a large bandwidth (e.g., Ultra-Wide bands (UWB) transmission). For opportunistic spectrum sharing, the signal of every system is transmitted via a virtual channel that is created by controlling the use of time and space in order to maintain the signal quality of the systems that share the frequency resources. For details on underlay spectrum sharing and opportunistic spectrum sharing, refer to Cited Reference 1 “A Tutorial on Ultrawideband Technology”, Doc. IEEE 802.15-00/082r0, March 2000, [searched on Feb. 27, 2006], URL: http://grouper.ieee.org/groups/802/15/pub/2000/Mar00/00082r1P802-15_WG-UWB-Tutorial-1-XtremeSpectrum.pdf”, and Cited Reference 2 “Wei Wang and Xin Liu, “List-Coloring based Channel Allocation for Open-Spectrum Wireless Networks,”, IEEE VTC 2005, [searched on Feb. 27, 2006], URL: http://www.cs.ucdavis.edu/˜liu/paper/open-spectrum(vtc05).pdf.
However, underlay spectrum sharing has a drawback owing to its limited transmission distance because the power is kept low. On the other hand, for opportunistic spectrum sharing, this suffers from the problem that the frequency resources might be underutilized, e.g., the case of two systems using the same frequency band by time sharing where one of the systems accesses the frequency band less frequent than the other system (resources are underutilized in time), and the case where those systems use different frequency bands simultaneously but one of the systems does not access its frequency band fully (resources are underutilized in frequency).
Therefore, in conventional systems, there is a limit on spectrum utilization efficiency; thus, it is necessary to further improve the usability of spectrum in future as spectrum demand is expected to increase and radio access schemes are also expected to diversify.